1. Field of the Invention
The present invention relates to an optical delay element using a photonic crystal.
2. Description of the Related Art
One of the subjects in optical communication is to provide an optical switching system able to change optical paths without opto-electric conversion of optical signals when transmitting optical packet data. In the related art, however, the optical signals have to be completely converted to electrical signals first, and then after the transmission paths are determined (changed) in form of electrical signals, the electrical signals are converted to optical signals again by electric-opto conversion.
In order to transmit optical signals themselves, it is necessary to use a switching element for changing the polarization direction of the light beam being transmitted, and an element for analyzing the polarization direction of the light beam. In addition, in an optical packet, a header containing path information may be appended prior to a section of actual information to sort the optical information. At least in principle, all these operations can be carried out by light, but the problem that should be solved is the switching element. Alternatively, it is also conceivable to change paths electrically, and change the optical paths without opto-electric conversion.
In order to conduct optical packet communications, fast optical path converting switches, wavelength converting elements, and optical delay elements are required. The optical delay elements serve to compensate for various signal delays required in path switching of optical signals. For example, background art of this technical field is disclosed in Japanese Laid-Open Patent Application No. 2003-156642 (referred to as “reference 1” hereinafter), Japanese Laid-Open Patent Application No. 2002-333536 (referred to as “reference 2” hereinafter), and Physical Review Letters Vol. 87, 253902 (2001) (referred to as “reference 3” hereinafter).
The current situations of these optical elements are described below.
First, as for the fast optical path converting switches, a switch operating at pico-second level is obtainable by applying an electric field to an optical waveguide switch which is constructed by combining optical modulation elements made of electrical-optical materials or semiconductor materials possessing a quantum-well structure. In addition, switches made of non-linear materials for optically switching optical paths are also proposed and being studied.
Concerning the wavelength converting elements, it is found that wavelength conversion can be implemented by making use of harmonic component generation phenomenon generated by non-linear materials, and research and development in this subject are being made.
As for the optical delay elements, in order to implement an optical delay element, a scheme as described below is adopted presently, in which an optical fiber of super-low loss is used to generate delay of optical pulses.
However, in this scheme, in order to delay the optical pulses by 1 ns, a quite long optical fiber has to be used, for example, for an optical fiber having a refractive index of 1.5, the required length is 20 cm to generate a delay of 1 ns. Additionally, when transmitting single wavelength optical signals, a number of optical fibers equaling the number of transmission channels have to be prepared. For these reasons, the optical device requires large size, and it is difficult to make the optical device compact.
The following solutions to this problem are proposed. First, in principle, because it is in the optical path that delay of the optical pulses occurs, if the refractive index of the transmission path is increased, the size of the transmission path can be reduced accordingly. Here, the refractive index of the transmission path is determined by a so-called “group refractive index”, which influences the optical pulses. For example, when the group refractive index is 10, corresponding length of the optical delay element is 3 cm, and when the group refractive index is 30, the length of the optical delay element can be reduced to 1 cm, so that the device is greatly reduced in size. This greatly contributes to realization of multi-channel optical packet communications.
Generally, the refractive index of a material is equal to the group refractive index, but they may differ from each other for some special materials or special structures.
Materials producing a very large group refractive index on optical pulses at low loss are still not found so far. Meanwhile, it is well known that when optical pulses are transmitted in an optical waveguide of a multiple reflection structure, the optical pulses have a large group refractive index. Especially, it is reported recently that when the optical pulses are transmitted in an optical waveguide constructed by using a so-called photonic crystal, which has a dielectric periodic structure, a large group refractive index as great as 50 is observed. For example, this phenomenon is reported in the aforesaid reference 3.
In the photonic crystal, a dielectric periodical structure is formed artificially at the level of the period of light, thus forming a band structure for photons. This structure is analogous with the band structure for electrons in many aspects, specifically, a so-called “photonic bandgap” is formed because of the periodic structure in the photonic crystal, which is a forbidden band of photons. Because of strong optical confinement effect and extraordinary dispersion effect, the photonic crystal has various characteristics, which are not attainable by usual optical delay elements. Therefore, it is expected that optical devices constructed using the photonic crystal will be essential devices in constructing photonic ICs having sizes much smaller compared to the present optical integrated circuits.
Although it is ideal that the photonic crystal be constructed to have a three dimensional structure, the characteristics of the photonic crystal can also be obtained with a two-dimensional photonic crystal formed in a plane at one time by using semiconductor process techniques. Such a two-dimensional photonic crystal can be formed in the following way. That is, a thin film of a high dielectric constant is sandwiched with a material of a low dielectric constant or a film of a high reflection rate, such as a metal film, thereby, realizing light confinement, and a photonic crystal structure is formed from holes or pillars in the thin film. Such a structure can be constructed by an in-plane fine processing technique; thus, the two-dimensional photonic crystal can be fabricated by using semiconductor chip process techniques.
A two-dimensional photonic crystal formed by arranging a thin film having a large refractive index on a substrate having a small refractive index is called a “two-dimensional photonic crystal slab”, and it is expected that the two-dimensional photonic crystal slab will be able to show satisfactory characteristics of the photonic crystal if the ratio of the refractive index n1 of the thin film to the refractive index n2 of the substrate is not less than or equal to 2 (that is, n2/n1>2). As examples of the two-dimensional photonic crystal slab, there is an air-bridge structure in which holes constituting a photonic crystal arrangement are formed in a thin film having a large refractive index (n>2), and the thin film is suspended in the air. In addition, the photonic crystal structure can also be formed on a substrate, which is constructed by arranging a semiconductor thin film (n=3 to 3.5) on a SiO2 substrate having a relatively small refractive index (n=1.5).
In these years, an SOI (silicon-on-insulator) substrate is used to improve performance of electronic devices. The SOI substrate is constructed by arranging a silicon thin film (n is approximately 3.5) on an SiO2 substrate, which is an insulator. In other words, the SOI substrate is highly suitable for fabricating the aforesaid two-dimensional photonic crystal slab, and is widely used in studies of the two-dimensional photonic crystal slab. For example, it is reported that a sharply bent optical waveguide has been fabricated with the SOI substrate.
Concerning the photonic crystal used as optical paths, a line defect optical waveguide is reported, in which a portion of the photonic crystal structure is removed. It is reported, in papers and in other ways, that a sharp bend and enhancement of the group refractive index are obtainable with the line defect optical waveguide made from the photonic crystal slab.
The line defect optical waveguide is characterized in that a large group refractive index and large wavelength dispersion of the group refractive index are obtainable at the same time. Making use of the large wavelength dispersion, it is easy to construct a dispersion compensation structure.
When using the line defect optical waveguide to delay optical pulses, however, if the wavelength dispersion of the group refractive index is too large, the refractive index varies even when the wavelength changes slightly, and this line defect optical waveguide is not suitable for use as an optical element.
In other words, when using the line defect optical waveguide to construct the optical delay element, it is required that the group refractive index be sufficiently large, but the wavelength dispersion of the group refractive index be approximately constant in a band of practical use, and it is preferable that the band for practical use be as wide as possible.
The aforesaid reference 1 discloses an invention in which the width of the line defect optical waveguide is narrowed, but this invention is devised to reduce the group refractive index, hence it is not applicable to an optical delay element.